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But O' Neill's expertise at restructuring sick banks did not serve him well in 2009, when he came close to being
Bank of America's(BAC - Get Report) CEO, only to lose out to Brian Moynihan.

According to media reports, O' Neill was pushing for a break up of Bank of America to make it simpler and less prone to volatility, but the bank preferred to stay the course as a money center bank.

Now as the chairman of Citigroup, O'Neill might have an easier time pushing for a more focused business model, given CEO Vikram Pandit's goal of returning the bank to a more traditional banking model that is focused on lending.

O' Neill was appointed to Citi's board in 2009 and oversaw the winding down of the bank's unwanted assets as a member of the executive committee of Citi Holdings, which houses non-core assets.

In the fall of 2011, he was named chairman of Citibank N.A.

Hedge fund manager Tom Brown said O' Neill had been instrumental in setting up the bank structure of the new Citigroup and had high praise for the new chairman. "We say there are 7,000 banks, but not 7,000 great bank CEOs.Michael o' Neill is one of the great bank CEOs," he told
Bloomberg Television.. While acknowledging that O'Neill was now a chairman and not a CEO, he asked "How many people when they get to be named CEO of a bank does their stock rise 10%? That's what happened with Bank of Hawaii."

CLSA analyst Mike Mayo had called for
Richard Parsons departure
a few weeks ago, arguing that that the bank needed a new chairman who would better represent shareholders' interests.

"If a change were to occur, our hope would be that the new person would be more than a figurehead, acting as somebody who could better "mind the store" and act as a true agent on behalf of investors," Mayo wrote.